Production of medium carbon ferrochromium



United States Patent Ofifice 3,085,604 PRODUCTIQN F MEDIUM CARBON FERROCHROMIUB'I Heinrich W. Rathmann, Cambridge, Ohio, and John C. Wolfe, New Haven, W. Va., assignors to Vanadium Corporation of America, New York, N.Y., a corporation of Delaware N0 Drawing. Filed Dec. 26, 1961, Ser. No. 162,243 1 Claim. (Cl. 75-1305) This invention relates to the production of a medium carbon ferrochromium containing carbon in the range of from 3.5% to less than 5%, silicon below a maximum of about 1.50%, impurities, such as titanium and sulphur being at low levels. Such a product is needed in the manufacture of a variety of alloy steels, particularly constructional alloy steels, tool steels, stainless steels, etc.

Considerable difiiculties have been experienced hitherto in the production of this grade of ferrochrornium, since the direct smelting of chromite with carbon leads to a carbon content of 5 to 8%, and sometimes even higher. However, industry desires a carbon content of between 3.5% and less than 5%, and a silicon content under about 1.5%. A number of processes have been developed in order to reduce carbon to this level, starting with a ferrochromium directly smelted from ore.

It is known that the level of silicon in a ferrous metal determines the level of carbon that can be present. Thus the higher the silicon, the lower the carbon that can be present in the metal. Thus one of the methods now used consists in producing by direct smelting a highsilicon ferrochromium with about 5% or more C which is then reacted with a chrome ore-lime melt. In this reaction the silicon in the silicon-containing ferrochromium reduces the chromium oxide contained in the chrome orelime melt. The chromium thus produced passes into the feirochromium, while oxidized silicon passes from the ferroohromium into the slag as silica.

In practice a 15% silicon-containing ferrochromium is used having approximately 5% C. When the silicon contained in such alloy is allowed to react with chromic oxide, sufiicient chromium is introduced into the melt to reduce the carbon level in the final product to the desired range below 5%.

These factors have been well recognized by those skilled in the art, however, considerable difiicu'lties have been experienced in carrying out the process in practice. Since the ore-lime melt is lighter in weight by a sub-stantial margin than the silicon-containing ferrroehromium, it has been the practice to teem the metal into the orelime melt and to permit a reaction to take place under conditions insuring an intimate mixing of the two reagents. It was found, however, that the extensive foaming pro duced in this reaction prevented a successful performance of this operation, or at least limited the scale of the process by imposing the necessity of keeping the weight of reagents so small, that the process became marginal in economy.

In order to counter the shortcomings of this approach, an alternative practice was developed under which the molten silicon-containing ferrochromium was teemed into the ore-lime melt in the furnace while maintaining the electric arc. Thereby, foaming was minimized, but refractory costs became high and the time required for the reaction to take place was relatively long. Consequently, these factors rendered this approach also uneconomic.

It is .an object of this invention to produce a medium carbon ferroch-romium of great cleanliness by reacting silicon-containing ferrochromium with a chrome ore-lime melt.

It has been the experience that reactions occurring at a metal-slag interface, i.e. the metal being contacted by a supernatant layer of slag, are slow and require additional heat for successful performance. Such a process is for instance, embodied in the steel making operation by the open hearth process. It is precisely the slowness of open hearth steel making practice which is currently causing its displacement by or modification with oxygen steel making practices.

It was thus an unexpected result to find that, in accordance with our invention, the teeming of the ore lime melt upon the silicon-containing ferrochromium in a ladle produced the desired result by sustaining a rapid, but not excessively violent, and complete reaction between the silicon in the silicon-containing ferrochromium and the chromic oxide in the chrome ore-lime layer.

The reaction is quite economic with respect to scope, i.e. Weight of reagents that can be brought together, and to the yield, in that the silicon-containing ferrochromium absorbs a substantial proportion of the chromium value-s contained in the chrome ore-lime melt, while the silicon is oxidized out of the ferrochromium, and the resultant discard slag is quite lean in respect to chromium.

The process of this invention involves the following steps: Production of a silicon-containing ferrochromium containing between about 4.5 and 6% C, between about 310 and 18% Si and between about 52 and Cr, the balance being iron. Secondly, a chrome ore-lime melt is produced, preferably from pebble lime and chrome ore concentrate, the chrome ore-lime melt containing between about 20 and 30% C-r O and 6 to 10% FeO derived from the ore constituent which may carry also alumina, magnesia and silica, balance CaO and/or MgO or other basic oxides as constituents in the lime-portion of the melt. A typical composition of the ore-lime melt is as follows: 29% Cr O 9% FeO, 43% C210, 10% MgO, 7% A1 0 2% SiO;. .Thirdly, the silicon-containing ferroch-romium is teemed into a ladle and the chrome orelime melt is poured on top of the metal, causing an exothermic reduction reaction. The ladle is allowed to stand for a time suflic-ient to insure that the reaction between the chrome ore-lime melt and the silicon in the siliconcontaining ferrochromium has gone substantially to completion. In some cases it may be necessary to resort to reladling in order to agitate the metal and slag. In either case, the metal is tapped from a bottom tap hole and poured into molds where it is allowed to solidify. The slag which is quite lean with respect to chromium and iron is discarded. The silicon contained in the siliconcontaining ferrochromium is substantially completely consumed in the reaction, in some cases to such an extent that some ferrosilicon must be added to the alloy when substantial silicon levels are specified.

In practice this process causes a rapid, and very active, but controllable reaction which commences immediately upon pouring the high C1' O chrome ore-lime melt onto the silicon-containing ferrochromium. This exothermic reaction is controlled by the mate of addition of the chrome ore-lime melt and by employing a ladle the dimensions of which are suitably related to the amounts of siliconcontaining fer-rochromium and chrome ore-lime melt employed. It is advisable to perform this operation in the ladle holding the metal on a weighing scale so that the addition of the chrome ore-lime melt can be accurately gauged until the specified amount has been added.

We have found that, where the silicon-containing ferrochromium contains between and 18% silicon and the chrome ore-lime melt contains between 20' and 30% Cr O and between 6 and 10% of FeO, in order for the reaction between the silicon-containing ferrochromium and the chrome ore-lime melt to be adequately rapid while not excessively violent, it is critical that during the reaction there be provided an interfacial contact area between the silicon-c'ontain'in'g ferrochromium and the chrome ore-lime melt of between 1.0 and 2.7 square feet per cubic foot of ferrochromium. If this interfacial contact area is less than 1.0 square foot per cubic foot of ferrochromium, the reaction is not adequately rapid. On

the other hand, if this interfacial contact area is greater than 2.7 square feet per cubic foot of ferrochromiurn, the reaction is excessively violent.

The interfacial contact area between the silicon-containing ferr-ochromium and the chrome ore-lime melt is determined, for given weights of these two materials, by the dimensions of the ladle in which the reaction is carried out. Thus, for given weights of silicon-containing ferrochromium and the chrome ore-lime melt, a large diameter ladle will provide a large interfacial contact area per cubic foot of ferrochromium, and on the other hand, a small diameter ladle will provide a small interfacial contact area. We have found that where the charge of silicon-containing ferrochromium weighs about 5,000 pounds and the chrome ore-lime melt Weights about 7,000 pounds, a ladle having a bottom diameter of about 60", a top diameter of about 70", and a height of about 80" will provide an interfacial contact area within the limits above specified. If the charge of silicon-containing ferrochromium weighs about 12,000 pounds, and the chrome ore-lime melt weighs about 15,000 pounds, an interfacial contact area between the limits above specified will be provided if the ladle has a bottom diameter of about 100", a top diameter of about 120" and a height of about 100".

Smaller or larger quantities of the silicon-containing ferrochromium and the chrome ore-lime melt may be employed but the ladle dimensions must be such as to provide an interfacial contact area between the limits specified.

The ladles are lined with basic refractories. It is occasionally advisable to repour the ferrochromium and chrome ore-lime melt, i.e. to teem it from one ladle into another and back again, to facilitate intermingling of chrome ore-lime melt and ferrochromium, in order to complete the reaction between the Cr O in the chrome ore-lime melt and the silicon in the ferrochromium. Such a procedure becomes possible after the chrome ore-lime melt has become somewhat impoverished with respect to chromic oxide, and thus the reaction between silicon in the ferrochrornium and the oxide in the chrome ore-lime melt will no longer be so violent as to cause excessive foaming.

Sometimes also a simple repouring of the mixture into a second vessel is resorted to, after the calculated amount of chrome ore-lime melt has been added to the siliconcontaining ferrochromium, to assure the substantial removal of the silicon.

The following example further illustrates the invention.

Example A high carbon silicon-containing ferrochromium was There were 10 charges for each tap.

The silicon-containing ferrochromium product Weighed 6,750 pounds and analyzed as follows:

Percent Chromium 5 9.5 0 Silicon 13 .5 6 Carbon 5 .27 Iron Balance Simultaneously 4,700 pounds of Rhodesian chrome ore concentrates were melted together with 3,800 pounds of pebble lime resulting in 8,500 pounds of a chrome orelime melt which analyzed 29.23% Cr O and 8.49% FeO, balance largely CaO. The silicon-containing fen'ochromium was teemed into a ladle which was placed on scales and the chrome orelime melt was slowly added to it. The ladle had a bottom diameter of 76", a top diameter of and a height of 84". Its volume was 290 cubic feet. The interfacial contact area between the silicon-containing ferrochromium and the chrome ore-lime melt was about 2 square feet per cubic foot of ferrochromium. The time cycle of the react-ion from start to finish was about 10 minutes, 35 minutes were required from opening of the tap hole of the silicon-containing ferrochromium furnace to the casting of the product. The final alloy weighed 8,700 pounds, which analyzed as follows:

. Percent Chromium 69.35 Silicon 1.00 Carbon 4.16 Titanium 0.005 Sulphur 0.009 Iron Balance The slag contained 6.0% Cr O and 2.5% FeO.

The invention is not limited to the preferred embodiment, but may be otherwise embodied or practiced within the scope of the following claim.

We claim:-

The method of making ferrochromium containing from about 3.5 toless than 5% carbon, and less than about 1.5% silicon, which comprises providing in a ladle a molten bath of silicon-containing ferrochromium containing from 4.5 to 6% carbon, from 10 to 18% silicon and from 52 to 65% chromium, balance substantially all iron, pouring a chrome ore-lime melt onto said molten bath of silicon-containing ferrochromiumand allowing them to react, the chrome ore-lime melt containing about 20 to 30% Cr O and about 6 to 10% FeO, the silicon-containing ferrochromium and the chrome ore-lime melt being in such proportions as to reduce the silicon content of the ferro-chromium to less than about 1.5 the dimensions of the ladle in which the reaction occurs being so related to the quantities of silicon-containing ferrochromiurn and chrome ore-lime melt that there is provided an interfacial con-tact area between the silicon-containing ferrochromium and the chrome orelime melt of between 1.0 and 2.7 square feet per cubic foot of ferrochromium.

References Cited in the file of this patent UNITED STATES PATENTS 1,538,893 Hamilton May 26, 1925 1,994,679 Arness Mar. 19, 1935 2,276,074 Vignos Mar. 10, 1942 

